Solar modules produced with thin-film technology are based on semiconductors that are applied preferably over a large surface area of usually inexpensive support layers such as glass, metal foil or plastic films. The various technological variants have not yet been developed to the stage of serial production as is the case with silicon-wafer technology, but over the medium-range, they entail a greater cost-reduction potential. Expensive semiconductor material is saved here in comparison to conventional wafer technology. This is particularly important in the case of crystalline silicon since the production capacities for silicon are currently not sufficient to meet the demand. Moreover, with thin-film solar modules, there is no need to assemble individual solar cells. The thin-film packets deposited over a large surface area are divided into smaller areas (usually into parallel, strip-like solar cell areas) and subsequently connected in series so as to be integrated. In this manner, support layers measuring up to one square meter can be coated, which significantly reduces the handling work at the factory. The challenge with thin-film solar modules lies in achieving sufficient levels of efficiency through adequate light absorption in the functional layers.
The solar cell areas of the thin-film solar modules convert light into electric energy. Normally, the modules are made of direct or indirect semiconductor materials that, through doping, contain layer or areas having different conductivity for positive (p, holes, p-type) and negative (n, electrons, n-type) charge carriers. These layers will be referred to below as dopant layers for collecting the charge carriers. Positive and negative excess charge carriers generated by incident light are created mainly in the absorber layer, the latter being undoped (intrinsic, i) or only weakly doped in the case of a pin configuration, and being in contact on both sides with dopant layers that are highly n-type conductive as well as highly p-type conductive. If the absorber layer is doped, the at least one layer that is highly counter-doped is referred to as the emitter layer, and a p-n configuration is present. Optionally, the absorber layer and the emitter layer can also be in contact with functional layers that are highly doped with the same type. Field-passivation layers are formed that serve to backscatter uncollected charge carriers on the front or back (BSF or FSF layers or areas). The excess charge carriers are separated either at the p-n junction between the emitter layer and the absorber layer (p-n configuration employing a doped absorber layer), or they are separated by the electric field in the intrinsic or weakly doped absorber layer, said electric field being encompassed by the n-type and p-type dopant layers arranged on both sides of the absorber layer (pin configuration employing an intrinsic absorber layer), and they can be collected by contact systems that are electrically conductively connected to the associated areas or layers and can then be dissipated. The pin configuration can be seen as a limit case of the p-n configuration having a specific doping profile. However, the only excess charge carriers that contribute to the useful electric output of thin-film solar modules are those that actually reach the contact systems and do not recombine before that.
In contrast to conventional mono-crystalline or multi-crystalline solar cells made of silicon wafers, thin-film solar cells having an ultra-thin absorber layer are thinner by a factor of 100. Different industrial production methods ranging from vapor deposition of the support material in a high vacuum to sputtering methods are available for the various solar cell materials. It is expected that thin-film solar cells will account for considerable price reductions over the long run. Material savings, research into new semiconductor materials, low-temperature processes that are substantially more energy-efficient, simple module production through the structuring of full-surface solar cell areas and a high degree of automation all translate into lower production costs. Aside from the thin-film solar cells made of amorphous silicon (a-Si:H) or chalcogenide compound semiconductors (CI(G)S(e), CdTe), especially the thin-film solar cells consisting of micro-crystalline or poly-crystalline silicon (μc-Si, poly-c-Si wherein c-Si stands for both kinds) will be very attractive over the long run since they have the potential to attain high levels of efficiency, they are environmentally safe and the starting material is available in sufficient quantities. A particularly promising development is also the combination of micro-crystalline or poly-crystalline silicon as the absorber and amorphous silicon as the emitter, since, due to the hydrogen contained in it, its band gap is greater than that of crystalline silicon (c-Si/a-Si:H), and a-Si:H can passivate the absorber boundary surface very well.
International patent application no. WO 03/019674 A1 describes a thin-layer solar module having a back configuration of both contact systems (see FIGS. 1A, 1B pertaining to the state of the art, referred to below as “double-point concept”; see also Publication I by P. A. Basore: “Simplified Processing and Improved Efficiency of Crystalline Silicon on Glass Modules”, Proc. EPVSEC-19, Paris, France, June 2004). In this double-point concept, both back contact systems are configured with punctiform contacts having different structuring. Thus, in order to achieve an efficient collection of the excess charge carriers, structural sizes are needed that are substantially smaller than the effective diffusion length of the absorber layer. Consequently, very small structural sizes are necessary in the case of a worse absorber quality. At the present time, such structural sizes can only be achieved—if at all—with very complex technology, for example, by means of photolithography. Consequently, in order to configure the effective diffusion length as large as possible, the prior-art double-point concept also makes use of re-crystallized silicon for the absorber layer, since such silicon displays a relatively large effective diffusion length for thin-film solar modules. Therefore, only absorbers that exhibit large diffusion lengths can be employed in the prior-art double-point concept.
Exclusively in the field of wafer-based (thick-film) solar cell technology, European patent no. DE 11 2005 002 592 T2 describes, for an absorber wafer, back contact with a first contact system having punctiform contacts and an outer contact layer and with a second contact system having exclusively a full-surface, inner contact layer for a p-n configuration with an emitter layer arranged on the back. Both contact systems are on top of each other on the side of the absorber wafer facing the emitter, and they are electrically insulated from each other by means of an insulating layer. The absorber wafer is contacted punctiformly by the first contact system, while the emitter layer is contacted over the entire surface. The practical implementation of this concept exclusively for wafer-based (thick-film) solar cells is described in Publication II by R. Stangl et al. titled “Planar Rear Emitter Back Contact Amorphous/Crystalline Silicon Heterojunction Solar Cells (RECASH/PRECASH)” (Paper IEEE-PVSEC-33. Conf. San Diego, Calif., United States, May 12 to 16, 2008, submitted on May 8, 2008). Both of these publications, however, do not contain any information about a possible module interconnection. Fundamentally, however, the contacting and module interconnection concepts found in wafer technology cannot be readily transferred to thin-film technology since the electronic conditions in the absorber layer and thus the boundary conditions and requirements for the contact systems that collect the charge carriers are fundamentally different. This is also the reason why inexpensive and efficient contacting systems for thin-film solar modules have not yet become commercially available.